On Fri, Aug 26, 2022 at 04:39:08PM +0200, David Hildenbrand wrote: > On 26.08.22 16:25, Peter Xu wrote: > > On Fri, Aug 26, 2022 at 12:49:37PM +0200, David Hildenbrand wrote: > >> On 24.08.22 00:11, Peter Xu wrote: > >>> Yu Zhao reported a bug after the commit "mm/swap: Add swp_offset_pfn() to > >>> fetch PFN from swap entry" added a check in swp_offset_pfn() for swap type [1]: > >>> > >>> kernel BUG at include/linux/swapops.h:117! > >>> CPU: 46 PID: 5245 Comm: EventManager_De Tainted: G S O L 6.0.0-dbg-DEV #2 > >>> RIP: 0010:pfn_swap_entry_to_page+0x72/0xf0 > >>> Code: c6 48 8b 36 48 83 fe ff 74 53 48 01 d1 48 83 c1 08 48 8b 09 f6 > >>> c1 01 75 7b 66 90 48 89 c1 48 8b 09 f6 c1 01 74 74 5d c3 eb 9e <0f> 0b > >>> 48 ba ff ff ff ff 03 00 00 00 eb ae a9 ff 0f 00 00 75 13 48 > >>> RSP: 0018:ffffa59e73fabb80 EFLAGS: 00010282 > >>> RAX: 00000000ffffffe8 RBX: 0c00000000000000 RCX: ffffcd5440000000 > >>> RDX: 1ffffffffff7a80a RSI: 0000000000000000 RDI: 0c0000000000042b > >>> RBP: ffffa59e73fabb80 R08: ffff9965ca6e8bb8 R09: 0000000000000000 > >>> R10: ffffffffa5a2f62d R11: 0000030b372e9fff R12: ffff997b79db5738 > >>> R13: 000000000000042b R14: 0c0000000000042b R15: 1ffffffffff7a80a > >>> FS: 00007f549d1bb700(0000) GS:ffff99d3cf680000(0000) knlGS:0000000000000000 > >>> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > >>> CR2: 0000440d035b3180 CR3: 0000002243176004 CR4: 00000000003706e0 > >>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 > >>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 > >>> Call Trace: > >>> <TASK> > >>> change_pte_range+0x36e/0x880 > >>> change_p4d_range+0x2e8/0x670 > >>> change_protection_range+0x14e/0x2c0 > >>> mprotect_fixup+0x1ee/0x330 > >>> do_mprotect_pkey+0x34c/0x440 > >>> __x64_sys_mprotect+0x1d/0x30 > >>> > >>> It triggers because pfn_swap_entry_to_page() could be called upon e.g. a > >>> genuine swap entry. > >>> > >>> Fix it by only calling it when it's a write migration entry where the page* > >>> is used. > >>> > >>> [1] https://lore.kernel.org/lkml/CAOUHufaVC2Za-p8m0aiHw6YkheDcrO-C3wRGixwDS32VTS+k1w@xxxxxxxxxxxxxx/ > >>> > >>> Fixes: 6c287605fd56 ("mm: remember exclusively mapped anonymous pages with PG_anon_exclusive") > >>> Cc: David Hildenbrand <david@xxxxxxxxxx> > >>> Cc: <stable@xxxxxxxxxxxxxxx> > >>> Reported-by: Yu Zhao <yuzhao@xxxxxxxxxx> > >>> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx> > >>> --- > >>> mm/mprotect.c | 3 ++- > >>> 1 file changed, 2 insertions(+), 1 deletion(-) > >>> > >>> diff --git a/mm/mprotect.c b/mm/mprotect.c > >>> index f2b9b1da9083..4549f5945ebe 100644 > >>> --- a/mm/mprotect.c > >>> +++ b/mm/mprotect.c > >>> @@ -203,10 +203,11 @@ static unsigned long change_pte_range(struct mmu_gather *tlb, > >>> pages++; > >>> } else if (is_swap_pte(oldpte)) { > >>> swp_entry_t entry = pte_to_swp_entry(oldpte); > >>> - struct page *page = pfn_swap_entry_to_page(entry); > >>> pte_t newpte; > >>> > >>> if (is_writable_migration_entry(entry)) { > >>> + struct page *page = pfn_swap_entry_to_page(entry); > >>> + > >>> /* > >>> * A protection check is difficult so > >>> * just be safe and disable write > >> > >> > >> Stumbling over the THP code, I was wondering if we also want to adjust change_huge_pmd() > >> and hugetlb_change_protection. There are no actual swap entries, so I assume we're fine. > >> > >> > >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c > >> index 482c1826e723..466364e7fc5f 100644 > >> --- a/mm/huge_memory.c > >> +++ b/mm/huge_memory.c > >> @@ -1798,10 +1798,10 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma, > >> #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION > >> if (is_swap_pmd(*pmd)) { > >> swp_entry_t entry = pmd_to_swp_entry(*pmd); > >> - struct page *page = pfn_swap_entry_to_page(entry); > >> > >> VM_BUG_ON(!is_pmd_migration_entry(*pmd)); > >> if (is_writable_migration_entry(entry)) { > >> + struct page *page = pfn_swap_entry_to_page(entry); > >> pmd_t newpmd; > >> /* > >> * A protection check is difficult so > >> diff --git a/mm/hugetlb.c b/mm/hugetlb.c > >> index 2480ba627aa5..559465fae5cd 100644 > >> --- a/mm/hugetlb.c > >> +++ b/mm/hugetlb.c > >> @@ -6370,9 +6370,9 @@ unsigned long hugetlb_change_protection(struct vm_area_struct *vma, > >> } > >> if (unlikely(is_hugetlb_entry_migration(pte))) { > >> swp_entry_t entry = pte_to_swp_entry(pte); > >> - struct page *page = pfn_swap_entry_to_page(entry); > >> > >> if (!is_readable_migration_entry(entry)) { > >> + struct page *page = pfn_swap_entry_to_page(entry); > >> pte_t newpte; > >> > >> if (PageAnon(page)) > >> > >> > >> @Peter, what's your thought? > > > > IMHO they're not needed? > > > > The rule is simple in my mind: we should only pass in a pfn-typed swap > > entry into pfn_swap_entry_to_page() (or the new swp_offset_pfn()), or it's > > a violation of the API. In these two cases they do not violate the API and > > they're always safe because they're guaranteed to be pfn swap entries when > > calling. > > I was wondering about extreme corner cases regarding the struct page. > > Assume we have a hwpoison_entry that pointed at a valid struct page. We > can succeed in offlining+removing the section it's located on (I was > recently challenging if we want to keep that behavior as it's really > shaky already), freeing the relevant memmap entry and the memory section. > > pfn_swap_entry_to_page() -> pfn_to_page() would be problematic if there > is no memmap anymore. > > > I assume it's ok to always call it for is_pfn_swap_entry(), but in the > PMD case we only check for is_swap_pmd()? Isn't that problematic? I don't know extensively enough on hwpoison on validity of fetching page from pfn inside on online/offline ops, but.. if the only concern is about hwpoison entry existance here I think its fine? Because iirc we'l split thp when any of the subpage got poisoned, so we should never hit a hwpoison entry in thp path. > > > I was confused by the hugetlb case, it's indeed fine as we check for > is_hugetlb_entry_migration(). Right, it's more straightforward in the hugetlb case. -- Peter Xu
